Method for forming anodic oxide coatings having improved adhesive properties

ABSTRACT

A METHOD FOR IMPROVING THE ADHESIVE QUALITIES OF AN ANODICALLY FORMED COATING ON ALUMINUM SUBSTRATE COMPRISING SUBJECTING AN ALUMINUM ARTICLE TO A CURRENT DESITY OF FROM 20 TO 75 AMPS/FT.2 FOR A PERIOD OF FROM ONE SECOND TO ONE MINUTE IN AN ELECTROLYTE SELECTED FROM THE GROUP CONSISTING OF AN AQUEOUS SOLUTION OF OXALIC ACID AND AN AQUEOUS SOLUTION OF PHOSPHORIC ACID, SAID SOLUTIONS CONTAINING MORE THAN 10 GRAMS/LITER OF ALUMINUM. THE PROCESS FORMS PSEUDODBOEHMITE ON AN ANODIC OXIDE COATING WHICH PROVIDES THE IMPROVED ADHESIVE QUALITIES.

United States Patent 3,714,001 METHOD FOR FORMING ANODIC OXIDE COATINGS HAVING IMPROVED ADHESIVE PROPERTIES Geoffrey A. Dorsey, .lr., Danville, Califi, assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

No Drawing. Continuation-impart of application Ser. No. 22,022, Mar. 23, 1970, now Patent No. 3,672,972, which is a continnation-in-part of abandoned application Ser. No. 707,962, Feb. 26, 1968. This application Jan. 31, 1972, Ser. No. 222,383

Int. Cl. C23f 9/02, 17/00 US. Cl. 204-58 7 Claims ABSTRACT OF THE DISCLOSURE A method for improving the adhesive qualities of an anodically formed coating on aluminum substrate comprising subjecting an aluminum article to a current density of from 20 to 75 amps/ft. for a period of from one second to one minute in an electrolyte selected from the group consisting of an aqueous solution of oxalic acid and an aqueous solution of phosphoric acid, said solutions containing more than 10 grams/liter of aluminum. The process forms pseudoboehmite on an anodic oxide coating which provides the improved adhesive qualities.

BACKGROUND OF THE INVENTION Although aluminum is a corrosion resistant and attractive metal, it is frequently desirable to coat it with other materials such as paints, lacquers or vanishes. It may also be desirable to bond other materials such as plastics and metals to aluminum or to bond aluminum articles together with organic adhesives. In all of these cases it is necessary to treat the surface of the aluminum so that the coating material or the adhesive will adhere strongly to the aluminum substrate. As used herein, the term aluminum" includes pure aluminum, commercially pure aluminum and aluminum based alloys.

Various types of chemical conversion coatings have been developed over the years to provide the pretreatment necessary for bonding other materials to an aluminum substrate. A frequently used process involves treating the surface of aluminum with an aqueous solution of hexavalent chromium, phosphates and fluorides. This process is frequently used as a pretreatment process for paints. Other methods include chromating and phosphating processes. An excellent discussion of these processes is set forth in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Penner, 1964. However, the use of the chromates and fluorides in these various chemical conversion coatings presents a serious disposal problem, particularly in view of the increased concern over environmental pollution.

Other pretreatment methods such as conventional anodizing in an acidic electrolyte can be used but these are quite expensive. Moreover, the anodizing processes are not conducive for use in pretreating continuous lengths of aluminum because of the long times necessary for treatment.

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Another method of pretreatment prior to painting or the like involves coating the aluminum surface with a form of aluminum oxide known as pseudoboehmite. This form of aluminum oxide is a highly active form which differs slightly in structure from the normal boehmitic structure and is entirely dissimilar from the structure that is characteristic of conventional anodized coatings. From infrared absorption studies the pseudoboehmite family apparently contains a double bond between the aluminum and oxygen which can be detected within the IR regoin of 1300 to 1500 cm." in the bulk oxide. Apart from this double bond feature, pseudoboehmites in general also have lower cross linking properties than normal anodic aluminum oxides. The pseudoboehmites are so active that they react with or at least strongly associate with many other materials and are, therefore, capable of forming a strong bond between these materials and the aluminum substrate. This is discussed in U.S. Pat. 2,915,475 issued to Bugosh. In the Bugosh process, pseudoboehmite is formed separately from the aluminum article and then coated on the article. However, this process requires high temperatures which afifect the temper of the heat treated or work hardened aluminum material and further requires complicated process procedures which are time consuming and expensive.

Against this process the present invention was developed.

RELATED APPLICATIONS This Application is a continuation-in-part of copending application Ser. No. 22,022 filed Mar. 23, 1970, now US. Pat. 3,672,972, which is a continuation-in-part of application Ser. No. 707,962, filed Feb. 26, 1968, now abandoned.

DESCRIPTION OF INVENTION -It is the object of the present invention to improve the adhesive qualities of anodized aluminum to other materials by an anodic oxidation process wherein a pseudoboehmite is formed directly on the aluminum article during the anodic oxidation process. The process of this invention has several advantages. First, at least one processing step is eliminated because pseudoboehmite is formed directly on the aluminum article at the anode. Socondly, because of the short processing times of the present invention, it can be readily integrated into continuous process lines as a pretreatment for painting and the like. Moreover, the components of the bath of the present invention do not present serious disposal problems.

In accordance with this invention certain acids, namely, oxalic acid and phosphoric acid, are employed for anodizing which have long been known as acids suitable for producing porous anodic oxide coatings. However, it is not the object of this invention to produce the typical porous anodic oxide coating, but rather an anodic coating containing pseudoboehmite. To obtain the desired pseudoboehmite coating that adapts the aluminum article for coating with other materials, the anodizing must be carried out only under selected conditions in the acidic electrolytes. The above-described electrolytes are useful in that they are capable of exerting a dissolving action on the porous and barrier layer oxides, complexing or chelating aluminum, but still allowing anodic disassociation of the complex to redeposit the aluminum in the form of pseudoboehmite at the anode.

In addition to employing an electrolyte capable of forming an aluminum-containing anion, it is essential that the anodizing be effected under conditions at which aluminum oxide is formed on the article; the barrier layer or porous layer, or both, are dissolved rapidly in the electrolyte about as fast as they are being formed on the aluminum substrate to supply aluminum to the electrolyte; and pseudoboehmite is deposited from the solution to the anode. It is essential in the present invention that an initial oxide coating is formed prior to the pseudoboehmite deposiiton at the anode; however, the initial oxide coating can previously be formed in another anodizing process.

The aqueous electrolytes of the present inventIon contain from 100 to 1500 grams/liter, preferably 100 to 650 grams/liter, oxalic acid (C H O -H O) or 100 to 1000 grams/liter, preferably 100 to 500 grams/liter, of phosphoric acid (H PO The aluminum concentration in the electrolyte must exceed grams/liter, preferably more than gram/liter. The electrolyte temperature for the anodic process ranges from about 40 to 90 C., preferably from about 60 to 85 C. In the process, the aluminum workpiece is subjected as an anode in the electrolyte to a current density of from about to 75 amps/sq. ft. (DC) for a period of time from about one second to one minute. Up to grams/liter of sulfuric acid can be added to the electrolyte to increase bath conductivity, but the amount of sulfuric acid should not exceed a molar ratio greater than one to 10 of the oxalic acid of phosphoric acid, whichever the case may be.

The voltage required to maintain a current density within the prescribed range depends upon the electrolyte and also on the cell resistance due to the anode-cathode distance, and the resistance in lead lines, contact points, and the like. Typical values range from 2 to volts.

The aluminum in the electrolyte apparently exists as a complex with the oxalic acid or posphoric acidin the case of oxalic acid, trisoxalato aluminic acid. It is believed that in the presence of an electrical field (whose value is that of the applied voltage divided by the thickness of the primary phase barrier layer in angstrom units) the complexed aluminum moves into the immediate vicinity of the anode and disassociates, resulting in the deposit of the pseudoboehmite in the anodic coating. For this reason, the aluminum concentration does not have to be at the point of saturation.

The pseudoboehmite produced in an anodizing electrolyte in accordance with the present invention is indistinguishable from the pseudoboehmite formed by nonanodizing methods in that both types exhibit absorption peaks in the same IR regions. To determine whether or not pseudoboehmite is deposited on the surface of the anodized layer, two tests were devised in which agreement is shown to be 100% conclusive. The simplest test involves the use of a standard adhesive tape such as No. 600 Scotch Brand cellophane tape applied to the anodic oxide surface. If the adhesive on the tape adheres to the oxide layer when the tape is peeled back from the anodized coating, then pseudoboehmite is present. However, if no adhesive is present on the anodized coating when the tape is removed, pseudoboehmite is not present. The ultimate test for surface detection of pseudoboehmite makes use of infrared techniques for surface examination, such as spectral reflectance or ATR. With such technique, time consuming procedures are required. Nonetheless a 1:1 correlation is achieved between positive and negative tape test results and the independent ATR infrared examination. The tape test is more conveniently utilized and is solely employed in most cases described herein unless the results are questionable.

Aluminum can be initially added to the electrolyte in the form of salt such as aluminum oxalate, aluminum phosphate and the like. Alternatively, the electrolyte can be broken in by anodizing aluminum in the electrolyte until the aluminum concentration reaches the desired level. Large quantities of aluminum in solution do not detrimentally affect the process.

The following examples are presented to demonstrate embodiments of the present invention:

EXAMPLE 1 An aqueous electrolyte was prepared contaianing 378 grams/liter of oxalic acid (dihydrate) and about 25 grams/liter aluminum. Two samples of a 3105 aluminum alloy sheet (Aluminum Association designation) were cleaned by conventional means and anodized in this electrolyte at a temperature of C. One sample was anodized for 15 seconds and the other sample for 30 seconds at a current density of 50 amps/ft. The oxide layers formed by the anodizing process contained large quantities of pseudoboehmite. These samples were primed with an epoxy-based primer and then coated with an acrylic paint. The painted samples were subjected to accelerated corrosion testing such as the SWAACT tests (Seawater Acetic Acid Cyclic Test as Navy Specification QQ-A- 00250/20) and exhibited excellent corrosion resistance.

EXAMPLE 2 An aqueous electrolyte was prepared containing about 294 grams/liter of phosphoric acid and about 25 grams/ liter of aluminum. Two samples of an 1100 aluminum alloy sheet material were cleaned by conventional means and anodized in this electrolyte at a temperature of 80 C. One sample was anodized for 15 seconds and the other for 30 seconds at a current density of 50 amps/sq. ft. The oxide layers formed by the anodizing process contained large quantities of pseudoboehmite. The two samples were subsequently coated with an epoxy-based primer and then painted with the acrylic paint. Both samples were subjected to accelerated corrosion tests such as the SWAACT Test, and both exhibited excellent corrosion resistance.

In SWAACT tests with panels which had been similarly treated except that the primer was omitted, no significant deterioration was noted after 200 hours of treatment. Other tests involving panels which were coated with pseudoboehmite and which were bonded together by various adhesives, such as epoxy-based adhesives, further indicate the superior adhesive properties of the coatings of the present invention. It should be noted that with these adhesive tests failures from tensile testing of these materials involved cohesive failures which indicates that the bond between the adhesive and the aluminum substrate is stronger than at least one of the materials being bonded.

It is obvious that various modifications and improvements can be made to the present invention without departing from the spirit thereof and the scope of the appended claims.

What is claimed is:

1. A method of forming an anodic coating on an aluminum article having improved adhesive properties comprising subjecting said article to a direct current at a current density from about 20 to 75 amps/ft. for a period of from one second up to one minute in an electrolyte selected from the group consisting of an aqueous solution containing from 100 to 1500 grams/liter oxalic acid and an aqueous solution containing from 100 to 1000 grams/ liter of phosphoric acid, said electrolytes being at a temperature from about 40 to C. and containing in solution more than 10 grams/liter aluminum.

2. The method of claim 1 wherein the electrolyte is an aqueous solution containing from about to 650 grams/liter oxalic acid at a temperature from about 60 to 85 C.

3. The method of claim 2 wherein the electrolyte contains up to 30 grams/liter of sulphuric acid.

5 6 4. The method of claim 1 wherein the electrolyte is an References Cited aqueous solution containing from about 100 to 500 grams/ liter of phosphoric acid at a temperature from 60 to 85 C. FOREIGN FE 5. The method of claim 4 wherein the electrolyte con- 1,051,665 12/1966 Great 20456 R tains up to 30 grams/liter of sulphuric acid. 5 1,235,661 6/1971 Great Bntam 204-58 6. The method of claim 1 wherein the electrolyte contains more thtan 15 grams/ liter of aluminum. JOHN MACK Pnmary Exammer 7. The method of claim 1 wherein the aluminum ar- R. L. ANDREWS, Assistant Examiner ticle is subjected to a current density of from about 20 to 75 amps/ft. for a period of time from about 15 to 10 US Cl. XR 30 seconds. A 

